Personal monitoring is invariably carried out using passive dosemeters incorporating either photographic film or thermoluminescent dosemeters. However these dosemeters are being supplemented by electronic devices for use as a warning device in high dose rate areas. The electronic dosemeters usually incorporate either a miniature GM counter or a solid state semiconductor detector and are designed to measure deep dose equivalent for photons in the range of 60 KeV to 1.25 MeV. The integrated dose is digitally displayed and in some of the pocket dosemeters visual/oral preset dose alarm is also provided. Microprocessor based versions of electronic dosemeters are also commercially available with sophisticated data acquisition and storage capabilities. (See Delacroix, D., Guelin, M., Lyron, C. and Feraud, J. P. “Dosicard: on-site evaluation of a new individual dosimetry system”, Radiat. Prot. Dosim. 58(3), 193–199(1995). And Toshikazu, Yoshiyuki Nagase, Takeshi Ishikura, Eisuke Okamoto, and Yoshiteru Yoshida, “A high reliability Personal Alarm Dosemeter with a semiconductor detector”, Fuji Electric Co. Ltd., Tokyo 191, Japan).
U.S. Pat. No. 4,996,429 (1991) describes an instrument for measuring ionization radiation acting upon a person. It can be carried in the pocket of a garment. It has a radiation detector exposed to a wide angle of ionizing radiation and a display unit showing the radiation received by the instrument. The instrument operates on rechargeable battery, has high power consumption and is bulky.
U.S. Pat. No. 4,430,569 (1984) describes a pocket type radiation dosemeter and a charging circuit for recharging the battery used for its operation. It is a compact, light-weight, usable by the layman, but the dosemeter proper is of conventional design at that time. The charging circuit includes a shake-type electrostatic generator, a voltage doubler for integrating generator output voltage of one polarity, and a switch operated by external permanent magnet. This type of dosemeter is based on an ion chamber radiation detector and a quartz fibre electrometer. The dosemeter is read by observing the position of a quartz fibre over a nonlinear scale through an eye piece. However, it has poor resolution (>10 μSv), limited range and it is highly susceptible to vibration, shock and humidity. Further it has no easy-to-read digital display.
U.S. Pat. No. 4,857,739(1989) describes pocket personal radiation monitor of the “chirper” type. A battery powered high voltage power supply is used to generate and apply a high voltage bias to a G-M tube radiation sensor. The high voltage is monitored by a low-loss sensing network which generates a feedback signal to control the high voltage power supply such that the high voltage bias is recharged to +500 VDC when the current pulses of the sensor, generated by the detection of ionizing radiation events, discharges the high voltage bias to +450 VDC. During the high voltage recharge period an audio transducer is activated to produce an audible “chirp”. The rate of the “chirps” is controlled by rate at which the high voltage bias is recharged, which is proportional to the radiation field intensity to which the sensor is exposed. The “chirp” rate sensitivity is set to be approximately 1.5 (chirps/Min/mR/hr). The G-M tube sensor is used in a current sensing mode so that the device does not paralyze in a high radiation field. This instrument, uses GM counter as detector, which can provide only a qualitative indication of the radiation dose through chirps, no digital display of accumulated dose, not of pen-type, has higher power consumption and is bulky.
U.S. Pat. No. 5,132,543 (1992) describes an electronic pocket dosimeter based on a GM tube sensor. U.S. Pat. No. 4,608,655 (1986) describes a wrist watch dosimeter based on an expensive CdTe (Cadmium Telluride) radiation sensor.
U.S. Pat. No. 5,567,946 (1996) describes a pocket dosemeter but it is not of digital type.
EP 0581422 (1994) relates to a Particle Dosimeter comprising of diodes for measurement of equivalent radiation dose due to neutrons, protons, electrons and photons and hence not relevant to the current invention.
With the wide spread use of radio isotopes for a number of applications and the rapid growth of atomic energy programmes world wide, there has been a need for an accurate and low cost pocket type dosemeter provided with a convenient readout device. There are a number of such dosemeters commercially available. These dosemeters use Geiger-Mueller (GM) counters or specially fabricated P-I-N Si semiconductors as the detector and some ASIC (Application Specific Integrated Circuit) for processing the radiation induced signal from the detector and are generally expensive. The object of this invention is to develop a low cost pocket dosemeter using readily available components including the detector.
There are different types of dosemeters presently in the field. Each of these is having certain unique features. Some of these are:    (i) Geiger-Mueller (GM) counters or specially fabricated P-I-N Si semiconductor diodes as the detector and some ASIC (Application Specific Integrated Circuit) for processing the radiation induced signal from the detector and are generally expensive.    (ii) Use of low capacitance type planar diffusion type Si photodiodes or Si heterojunction diodes.    (iii) Use of reverse bias of several tens of volts for the detector to get sufficient sensitivity.    (iv) Use of elaborate zero suppression circuits to achieve acceptable background level counts.    (v) Usable up to a maximum radiation dose rate of 0.5 Sv/h. (Sv is an unit of radiation dose equivalent).
Some models in the market relevant to the dosemeter of the present invention are:    (a) (Electronics Corporation of India Ltd.) ECIL make pocket dosemeter model PD 4506 uses GM Tube as detector, has a sensitivity of 10 μSv per count (against the current claim of 1 μSv per count), bulky (160 mm×65 mm×25 mm), heavy (160 gms) and maximum dose range of only 10 mSv (against the current claim of >1 Sv). (See “Digital Dosimetry System”, Electronics Corporation of India Ltd. ECIL P.O., Hyderabad, 500762).    (b) Pulsecho Systems Pvt. Ltd., Model “Dosirad” is GM tube radiation detector based, bulky (125 mm×50 mm×25 mm) and heavy (175 gm). (“Dosirad”, Pulsecho Systems (Bombay) Pvt.Ltd., Unit 110, Nirmal Industrial Estate, Near Sion Fort, Sion(E), Bombay 400 022.)    (c) PLA Electro Appliances Pvt. Ltd., Models PDM 103 & PDM 221L are GM tube radiation detector based, dose range of up to 100 mSv only, bulky (120 mm×65 mm×23 mm) and heavy. (“Pocket Dosemeter” models PDM 221L & PDM 103, PLA Electro Appliances Pvt.Ltd., Thakor Estate, Kurla Kirol Road, Vidyavihar (W), Mumbai 400 086).    (d) “Personal Digital Dosemeter, Model 885”; Victoreen, Inc. (Cleveland, Ohio, 1989)    (e) “Alarm Pocket Dosemeter (APD)”, Panasonic, Matsushita Electric Trading Co., Ltd. (Osaka, Japan)    (f) Personal Electronic Dosimeter DMC 2000 XB, etc., MGP Instruments Inc., 5000 Highlands Parkway, Suite 150, Smyrna, Georgia 30082.    (g) Electronic Pocket Dosimeter, MyDOSE mini, Model PDM-102, Aloka Co. Ltd., 6-22-1, Mure, Mitaka-shi, Tokyo, 181–8622, Japan.Drawbacks of the Prior Art:
The main drawback of the prior art is that the dosemeters presently available in the market are very expensive as they make use of expensive radiation detectors and ASICs/microprocessors. All the prior art types with digital display respond to X and Gamma rays in the energy range of 60 keV to 1.25 Mev within ±25% to ±30%; ideally an uniform response for the entire energy range is desired.
The prior art types have limited linearity beyond a certain dose rate though linearity up to much higher dose rate levels is desirable for some applications.
Most of the pocket dosemeters are heavy to carry them on the person throughout days in and out. Another drawback is that the size of some of these pocket dosemeters is not so convenient to keep them on person in the normal dress pocket.
Some of the dosemeters have high power consumption requiring either bulky batteries to provide continuous operation for at least 300 hours or rechargeable batteries to facilitate 12–24 hour continuous operation requiring over night external charging of batteries.
Object:
The principal object of the invention is to make a dosemeter, which will have an uniform response for X and Gamma rays in the energy range of 60 keV to 1.25 Mev within ±15%, for personal use; to develop a dosemeter which is inexpensive, small in size, light in weight and convenient to wear.
Another object of the invention is to make it appropriately sensitive to low levels of radiation and at the same time respond linearly to high radiation exposure rates, which may be encountered in an accident situation.